Surface polysaccharides of pathogenic bacteria, including CPs or the O-specific polysaccharide (OSP) of lipopolysaccharides (LPS), function as both essential virulence factors and protective antigens. Covalent binding of these saccharides to medically useful proteins to form conjugates increases their immunogenicity and confers on them T cell dependence, making them suitable vaccines for infants and children. The O-SP of Shigella sonnei bound to recombinant non-toxic P. aeruginosa exoprotein A (rEPA) exhibited an efficacy of over 70 percent in young adults exposed to 6 to 14 percent attack rates. This conjugate and that of S. flexneri 2a bound to the succinylated exoprotein A (rEPA-succ) were safe and induced IgG antibodies to the homologous LPS in 1- to 4-year-olds. A randomized, blinded, Phase 3 study of the conjugates in 1- to 4-year-olds, with each conjugate serving as a control for the other, showed the vaccines to be safe. Immunogenicity and efficacy were age-related with little efficacy in 1- to 2-year-olds but with about 70 percent in 3- to 4-year-olds. As before, fold increases in antibody levels were similar to those for adults, but the actual achieved levels were lower. We have shown that synthetic oligosaccharides of S. dysenteriae type 1 O-SP, bound by their reducing end to a carrier protein (sun configuration), induced significantly higher antibody levels than conjugates of the native O-SP bound to protein by several point attachments (lattice configuration). Synthesis of S. sonnei oligosaccharides was not successful. Therefore, we isolated lowmolecular mass O-SP-core (O-SPC) fragments of the native O-SP and used them to bind to proteins by oxime linkages between the terminal Kdo residues of the reducing end core and the aminoxy linkers bound to either BSA or to non-toxic recombinant diphtheria toxin or C.difficile toxin B. The coupling reaction was performed at a neutral pH, at room temperature, and in a short time. Levels of antiS. sonnei LPS IgG induced by these conjugates in young outbred mice were significantly higher than those induced by the full-length O-SP conjugates. The O-SP of Plesiomonas shigelloides O17 is structurally identical to that of S. sonnei, but only the core structure of S. sonnei has been published. We investigated the core structure of P. shigelloides O17, including its linkage to the O-SP, by nuclear magnetic resonance (NMR) and mass spectroscopy. A core structure, different form the S. sonnei core, was assigned.